Add grid size examples.

docs/examples/styles/grid_size_both.css

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+Grid {
+    grid-size: 2 4;  /* (1)! */
+}
+
+Label {
+    border: round white;
+    content-align: center middle;
+    width: 100%;
+    height: 100%;
+}

docs/examples/styles/grid_size_both.py

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+from textual.app import App
+from textual.containers import Grid
+from textual.widgets import Label
+
+
+class MyApp(App):
+    def compose(self):
+        yield Grid(
+            Label("1"),
+            Label("2"),
+            Label("3"),
+            Label("4"),
+            Label("5"),
+        )
+
+
+app = MyApp(css_path="grid_size_both.css")

docs/examples/styles/grid_size_columns.css

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+Grid {
+    grid-size: 2;  /* (1)! */
+}
+
+Label {
+    border: round white;
+    content-align: center middle;
+    width: 100%;
+    height: 100%;
+}

docs/examples/styles/grid_size_columns.py

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+from textual.app import App
+from textual.containers import Grid
+from textual.widgets import Label
+
+
+class MyApp(App):
+    def compose(self):
+        yield Grid(
+            Label("1"),
+            Label("2"),
+            Label("3"),
+            Label("4"),
+            Label("5"),
+        )
+
+
+app = MyApp(css_path="grid_size_columns.css")